The simplest solution is flats. If people are going to live in large towns at all they must learn to live on top of one another. But the northern working people do not take kindly to flats; even where fiats exist they are contemptuously named 'tenements'. Almost everyone will tell you that he 'wants a house of his own', and apparently a house in the middle of an unbroken block of houses a hundred yards long seems to them more 'their own' than a flat situated in mid-air.
